Why are civilizations that never had contact with one another have so much in common?

All ancient civilizations have in common the harvesting of solar energy flows through agriculture (farming) and pastoralism (animal husbandry), which is more efficient than harvesting solar energy through nomadic hunting and gathering, and harvesting solar energy flows through agricultural and pastoralism can also be scaled up in a way that hunting and gathering cannot be scaled up.

Moreover, it is possible to make the transition from nomadic hunting and gathering to settled agricultural civilization, whereas it is probably not possible to make a direct transition from nomadic hunting and gathering to industrialized civilization, which is even more efficient in extracting energy flows from the biosphere than are agricultural civilizations, and industrialized civilization is scalable to even a greater extent than agricultural civilization. Thus we should understand the agricultural origins of civilization that all ancient civilizations have in common as the first kind of civilization that is possible given the conditions that obtain on Earth (and which probably obtain on all Earthlike planets with a biosphere).

Everything that can be done to organize agricultural production more efficiently, with more intensive farming techniques, expansion of the scale of farming, and greater organization of systematic agriculture, makes the harvesting of solar energy flows more effective and more efficient, putting greater energy flows at the service of human purposes. Thus we see the emergence of metallurgy, a pottery industry, other craft specializations, systems of written language, accounting, legal codes, and state-level organization of production and distribution, and everything else that we associate with civilization soon after the Neolithic Agricultural Revolution.

Absolute